The temperature of tanks is usually controlled by feeding a temperature controlling medium through channels that are formed of half pipes welded to the tank envelope. If the temperature control involves heating, it is common to use steam as the temperature controlling medium. Tanks of this type are disclosed in DE 1,223,132 B1, DE 8,800,536 U1, and DE 4,117,273 A1.
DE 1,960,929 U1 discloses a temperature controlled tank wherein part of the tank envelope itself is constituted by a helically wound tube of rectangular cross-section. The inner side of the tube part may plated by welding.
While the known designs provide efficient heat exchange between the temperature controlling medium and the tank content due to the fact that part of the channel profile is formed by the tank envelope itself, there are problems in other respects.
One problem resides in the considerable weld lengths between the half tubes and the tank envelope, which involve the danger of leakage. Leaking temperature controlling medium causes corrosion of the tank envelope and damage to the insulation surrounding the envelope and the medium flow channels.
Leakage is prone to occur not only when the tank is first manufactured; at that time, leakage is relatively easy to detect and repair. The situation is considerably more problematic if leakage occurs at a later date as a result of stress between the flow channels and the tank envelope. Stress may result from mechanical load in operation, which leads to transitory or permanent deformation of the tank envelope, or from local temperature differences, such as when hot steam is suddenly supplied to a cold tank due to malfunction of the supply system.
In addition, welding of different materials is undesirable or even impossible. This is particularly true if the tank is made of a special steel.
Furthermore, welding the channels to the envelope not only involves considerable manufacturing expenditure; with a tank which is no longer new, contamination and/or deformation of the envelope make welding difficult, so that older tanks can hardly be retrofitted with a heating or cooling system.